Recirculating jet mills are widely used for effecting reduction of solid materials to extremely fine particle sizes. Most jet mills fall into one or the other of two general types: the horizontal, or “pancake,” jet mill, and the loop-type jet mill, which is usually, but not necessarily, in the form of a vertical loop, and sometimes referred to as a “vertical” jet mill.
In the horizontal jet mill, raw material enters a hollow, circular, chamber where it comes into contact with high velocity jets of gas that enter the chamber through its periphery at angles such as to cause a circulating movement of gas and particles in the chamber. The jets of gas cause the particles to accelerate and collide with one another. The collisions result in a reduction in the sizes of the particles. The smaller particles exit the chamber through a central opening, while the larger particles continue to circulate until they are reduced by collision into smaller particles.
A typical loop jet mill comprises a conduit arranged to form a closed loop or toroid. Gas nozzles are provided in the periphery of the conduit for the introduction of streams of gas (usually air or steam) at high velocities. Gas and particles circulate within the conduit in a direction determined by the direction of the jets of gas that enter the conduit through the nozzles. Typically, the loop is somewhat elongated, and comprises a bottom section, which serves as a grinding chamber, and a curved upper section, which serves as a classifier. These sections are typically interconnected by conduit sections, referred to as “stacks”. The gas nozzles are usually located in the grinding chamber, and are directed in such a way as to produce a circulating flow of gas in one direction around the loop. A material feed port precedes the grinding chamber in the direction of circulating flow, for feeding raw material into the gas stream circulating within the mill. The raw material is carried into the grinding chamber where it encounters the high velocity gas streams. These gas streams cause the particles to collide with one another to effect pulverization. The particles are then carried upward by the gas stream to the classifier section at the top of the mill. There, because of the curvature of the conduit and the inertia of the particles, the larger particles which are heavier, are concentrated toward the outer periphery of the classifier section. An outlet port is provided at or near the downstream end of the classifier section. The outlet port is open toward the inside of the loop, and fine particles are carried out through the outlet port with the gas stream. These particles are then separated from the gas stream by conventional separators such as cyclone separators, bag collectors, and the like. In the meanwhile, the heavier particles are returned to the grinding chamber, and continue to circulate in the mill until they are reduced to a sufficiently fine size to be carried out through the outlet port.
In some jet mills, classification of particles is aided by a dynamic classifier, which includes a rotor in the classifier section of the mill. The rotor effects centrifugal separation of the lighter small particles from the heavier large particles, causing the small particles to exit the mill while permitting the heavier particle to continue to circulate until they are reduced to small particles.